1. Field of the Invention
This invention relates to a method and an apparatus for symbol synchronization. This invention particularly relates to a method and an apparatus for generating a symbol sync signal in an OFDM-signal demodulating system.
2. Description of the Related Art
Some of systems for transmitting digital information signals such as digital video signals in limited frequency bands employ orthogonal frequency division multiplexing (OFDM). Examples of digital information signals handled by such OFDM-based transmitting systems are multi-value modulation signals or 256-QAM (quadrature amplitude modulation) signals. In general, OFDM-based transmission is good in suppressing multipath effects and jamming signal effects. Further, the OFDM-based transmission has a good efficiency of frequency use.
Orthogonal frequency division multiplexing (OFDM) employs multiple carriers which are orthogonal with respect to each other. The multiple carriers are modulated in accordance with digital information pieces to be transmitted, respectively. The modulation-resultant multiple carriers are combined into an OFDM signal which has a form as a random signal. Generally, inverse fast Fourier transform (IFFT) is used in generating an OFDM signal. The "orthogonal" multiple carriers mean that the spectrums of carriers neighboring one carrier are null at the frequency of the latter carrier.
Data transmission based on OFDM is executed symbol by symbol. Each OFDM transmission symbol interval is composed of a guard interval and an effective symbol interval. The guard interval precedes the effective symbol interval. The guard interval is used for reducing multipath effects. The effective symbol interval is used for transmitting information (data). A part of information transmitted during the effective symbol interval is also transmitted during the guard interval.
A receiver side subjects a received signal to a windowing process for every symbol. Specifically, the receiver side discards a signal transmitted during a guard interval, and extracts and uses only a signal transmitted during an effective symbol interval. The signal transmitted during the effective symbol interval is demodulated and decoded into original data (original information) by data recovering processes including fast Fourier transform (FFT).
The receiver side implements symbol synchronization which generates a symbol sync signal equal in timing to every effective symbol interval related to a received signal. The windowing process uses the symbol sync signal as timing information of every effective symbol interval, thereby extracting a signal portion transmitted during the effective symbol interval. The accuracy of the timing information, that is, the accuracy of the timing of the extraction of a signal portion, affects the accuracy of the recovered original data.
In a first known way of providing symbol synchronization, a signal to be transmitted is divided into frames each having a predetermined number of 1-symbol-corresponding signal segments. In every frame, a first 1-symbol-corresponding signal segment is set to a null symbol state, and two subsequent 1-symbol-corresponding signal segments are set to form a chirp signal (a sine sweep signal). A receiver side detects a null symbol signal and a chirp signal for every frame, and generates a symbol sync signal in response to the detected null symbol signal and the detected chirp signal. In this case, since only one symbol sync information piece is available during every time interval corresponding to one frame, the symbol sync signal can be updated at a frame frequency equal to a relatively low frequency. Therefore, the symbol sync signal generated by the receiver side tends to be slow in responding to a change.
In a second known way of providing symbol synchronization, symbol sync information is assigned to one carrier among multiple carriers. A receiver side recovers the symbol sync information, and generates a symbol sync signal in response to the recovered symbol sync information. In this case, other carriers tend to interfere with the symbol-sync-information carrier, and conditions of the interference generally change in time domain. The interference as well as the interference condition change reduce the timing accuracy of the symbol sync signal generated by the receiver side.
A third known way of providing symbol synchronization uses the fact that a part of information transmitted during an effective symbol interval is also transmitted during a guard interval. This fact means that a transmitted signal has a symbol-based correlation. A receiver side detects the symbol-based correlation, and generates a symbol sync signal in response to the detected symbol-based correction. In this case, the timing accuracy of the symbol sync signal generated by the receiver side depends on a bit error rate which occurs during signal demodulating and decoding processes.